[adamtheturtle]

Model Target authentication helpers and shared tests still select credentials by concrete backend identity. Supply credentials through typed backend fixtures instead, and let token requests accept only the supplied credentials and service URL.

The adapter now owns ordinary, mock-only, and signed-request credential fixtures. The existing signing opt-in condition and skip reason move with them, so an opted-out real signing test skips before loading credentials. Shared Model Target assertions and request helpers no longer import or select VuforiaBackend; implementation setup remains in the directory fixtures. Remove the redundant token wrapper and the outdated plugin-order comment.

Part of #3738 and #3737, independent of the VuMark URL changes in #3754.

Validation:

• Python 3.15: 519 Model Target cases pass with 244 real-backend combinations skipped, using synthetic credentials and no retries. The collection and pass/skip counts match the previous suite.
• A separate consumer loads the same test functions and existing adapters, supplies its own ordinary/mock-only/signed credential fixtures with a different CAD URL, and uses a custom HTTP service URL: the same 519 cases pass with 244 skips across both mock implementations.
• A local policy probe selects the real credential-policy path without real-service setup. Without the signing flag it skips before credential lookup; with the flag it returns injected synthetic credentials. Neither probe makes live requests or consumes training allowance.
• The complete Model Target directory and prepared-request module have 100% local statement and branch coverage. The credential fixtures and moved signing-policy branches are also fully covered.
• Pre-commit and pre-push checks pass, including CI partition validation, Mypy, Pyright, public type completeness, Ty, Pyrefly, and check-manifest. Pylint passes on all eight changed files, and git diff --check passes.

No source package, dependency, workflow, or coverage gate changes. Full-suite hosted validation will run on this PR.